Syringes are commonly used in many different medical procedures to inject fluids into a patient. Syringes can be connected to needles, catheters, or simply used to administer oral medicines in fluid form. In some medical imaging procedures, syringes can be used to inject radiographically opaque contrast medium (also commonly referred to as contrast agents) into a patient's vascular system. For example, angiography is a medical imaging technique used to visualize the inside of blood vessels and organs of the body. In angiography, the contrast agent is injected into the vascular system through a catheter positioned near the area of interest within the patient's body. The contrast agent is added to the blood to make the vessels visible on the x-ray images taken once the contrast agent is administered.
The contrast agent is normally injected from a manually-operated syringe into an attached medical manifold, which provides fluidic connections to a catheter, contrast agent supply, and often a saline solution supply. Connections between the manifold, syringe and catheter are typically made with a threaded connector, typically a Luer Taper connector.
For coronary angiography, it is common for between six (6) and twelve (12) milliliters of contrast agent to be injected per angiogram image. Injection of the contrast agent should be done quickly, usually within 2 seconds, in order to ensure the proper density of contrast agent in the vessels for imagining. Each angiographic procedure typically involves multiple images, which require multiple injections of contract agent.
Operators injecting the contrast agent typically use syringe made of plastic, with ring handles on the barrel (or flanges) and a plunger actuated by the operator's thumb. Often, the plunger will have a ring on the end allowing forward and backward motion with only the operator's thumb. This type of syringe provides little mechanical assistance in delivery of the contrast agent through catheters, especially given the trend towards smaller and smaller catheter diameters.
Catheter diameter is often indicated in “French” size increments, which represent intervals of 0.3 millimeters. For example, a 6 French catheter has a nominal outside diameter of 2.0 millimeters, and a 7 French catheter has a nominal outside diameter of 2.3 millimeters. Inside diameters depend on the type of catheter. Catheters used for angiography are typically diagnostic catheters, with a 5 French diagnostic catheter having an inside diameter of 1.3 millimeters.
Smaller catheters have been introduced because of the benefits associated with a smaller puncture hole required to insert the catheters into the body; as the size of the puncture hole decreases, the risk of puncture site complications and the time required for the patient to recover decreases. These recently-introduced smaller diagnostic catheters, in 4 French and 5 French sizes, have narrower diameters; a 4 French catheter can be expected to have an inside diameter of approximately 1.1 millimeters and a 5 French catheter an inside diameter of approximately 1.2 millimeters. The inside diameter of a 4 French catheter can therefore be about 15% smaller than that of a 6 French catheter and about 30% smaller than that of a 7 French catheter.